Generally, atoms themselves don’t expand but the volume they

Generally, when all the three states of matter, including solid, gas and liquid, expand when heated, the atoms themselves don’t expand but the volume they take up does. When a solid is heated, the atoms vibrate faster about their fixed position. As a result, the relative increase in size is small, a phenomenon of this effect is the small gaps in railway lines that are left to prevent buckling as the tracks expand when heated by the sun.  Solid materials consist of particles that are close together with the energy of motion being dominated by the attraction energy. When a solid is heated, the change in temperature causes phase changes through processes including fusion (melting) where the solid is changed to liquid and vaporization, where the liquid is converted to gas. The phase change in solids depends on the energy imparted on the particular solid.When a solid is heated, it changes to liquid. This process requires heat energy although the temperature does not rise, as shown in the graph below:Figure 1: Heating curve of a soild material Ref: http://www.ewart.org.uk/science/structures/str4.htmIn this case, consideration is made on the energy equilibrium and physical changes when aluminum metal is subjected to heating. The physical changes that occur when aluminum metal is heated include expansion, increase in temperature and the change of state.ExpansionWhen heat is added to the aluminum metal, the particles gain energy and vibrate vigorously about the fixed positions resulting to separation. Due to this, expansion occurs. The expansion of aluminum is in three dimensions but in most cases the change in length is of importance. This change is found using the coefficient of linear expansion of aluminum of 0.000024 per Kelvin.II. Change in temperatureAs heat energy is added to the aluminum metal, the particles’ kinetic energy is increased and they move at higher speeds. The amount of temperature increase is dependent on the amount of heat added, the mass and its material. The specific heat of aluminum is given as 0.9 kJ/kgºC, defined as the amount of heat required to raise a kilogram of aluminum by one Kelvin.. The amount of heat released can then be found by the formula;Heat Released = mass (kg) of  AlSpecific heat capacity of AlChange in temperatureIII.  Change in stateThe change of state of aluminum from solid to liquid on heating accompanies no increase in the temperature. This is because the heat absorbed is used to separate the particles involving the latent heat of fusion. The latent heat of fusion for aluminum is 398 kJ/kg. The amount of heat released as latent heat can then be found by taking the product of the latent heat of fusion with the associated mass i.e.Heat Released = Latent heat of fusion of Al mass of Al (kg) Pure Aluminum does not respond to heat treatment. However, some aluminum alloys undergo hardening when exposed to high temperatures for long durations for example the Aluminum-copper alloy. The phase changes taking place in the alloy are as shown in figure 2 below:Figure 2: The Al-Cu phase diagram showing the steps in the age-hardening heat treatment and the microstructures that are produced. source: http://www.totalmateria.com/page.aspx?ID=CheckArticle&site=ktn&NM=235 In this precipitation hardening process, the three steps involved include: Solution Hardening where the alloy is heated above the solvus temperature and soaked until it produces a homogeneous mixture of ?-alpha solid solution and the ? precipitates are dissolved Quenching where the solid ? is cooled rapidly leading to the formation of  ? solid solution containing copper but not an equilibrium structure. At this stage, ? precipitates do not form since the atoms do not have time to diffuse to potential nucleation. Aging :- this is the third step in which the supersaturated ? solid solution is heated below the solvus temperature producing finely dispersed precipitates. The atoms diffuse short distances and the extra copper atoms diffuse to numerous nucleation sites leading to precipitates growth. The alloy is strengthened by the impediment of dislocation movement by the fine precipitates.The age hardening occurs through mechanisms such as; Coherency strain hardening; Chemical hardening; and Dispersion hardening.

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